Kaposi sarcoma (KS) is among the most common of HIV-associated malignancies worldwide, and a leading cause of cancer morbidity and mortality in sub-Saharan Africa (SSA). Although KS can occur in HIV-uninfected individuals, particularly in SSA ? termed ?endemic? KS ? HIV infection increases the risk of KS several thousand- fold. KS is a unique tumor whose pathogenesis involves chronic infection with HHV-8, disordered angiogenesis, and inflammation. Increasing evidence points to the importance of the interaction of these various components of the tumor microenviroment (TME) in driving tumorigenesis and response to treatment in a range of malignancies. However, our understanding of the composition and function of the KS TME is limited. Detailed study of the KS TME in vivo will elucidate mechanisms of KS pathogenesis, including the unique role of HIV infection, could elucidate fundamental aspects of tumor biology and help identify new therapeutic approaches. Since 2011 our team has been conducting a comprehensive prospective cohort study (?HIPPOS?) of KS in HIV+ and HIV- adults initiating treatment at the Uganda Cancer Institute in Kampala, Uganda. Subjects enrolled on this study provide blood samples and serial tumor biopsies and are followed for up to 1 year to rigorously define treatment response and clinical outcomes. Our results to date, based on study of over 160 KS subjects, suggest possible differences in the clinical outcome of HIV+ and HIV- KS subjects in Uganda, and have provided provocative preliminary data on the virology and immunology of KS in Uganda. Our team has recently developed and implemented a protocol for the isolation of viable single cells from KS tumors ? including both tumor cells, tumor-infiltrating lymphocytes (TIL), and macrophages ? that offers the unique opportunity to extend our studies of the role of HIV in the development and progression of KS to the single-cell level. In this application we propose to define the role of HIV in the KS tumor niche by performing comprehensive molecular profiling of isolated single tumor cells, TIL, and macrophages from serially acquired KS tumors from 20 HIV+ and 20 HIV- adults. The specific aims are: 1) To compare the transcriptional profile of KS tumor cells from HIV+ and HIV- subjects. 2) To perform serial transcriptional profiling of immune cells in KS tumors from HIV+ and HIV- adults, and to determine if changes in their transcriptional profiles are correlated with suppression of HIV and with treatment response. 3) To define the antigenic specificity of candidate CD8+ ?public? HHV-8-specific infiltrating T cells in KS tumors that are associated with superior treatment response. The results of these studies will provide unprecedented insights into the role of HIV in the pathogenesis of KS and its response to treatment and could lay a foundation for antigen-specific immunotherapy of KS.